X-ray tube cathode



May 27, 1'958 z. J. ATLEE 2,836,747

x-RAY TUBE CATHODE Filed March 2a, 1955 u MVN HHHHHH 0, Y /9 20 /8 'fINVENTOR. ZED J'. ATLEE @M1/MM ATTORNE K2 United States Patent X-RAYTUBE CATHoDE Zed 3. Atlee, Chicago, Ill., assignor to Dunlee Corpi,Chicago, Ill., a corporation of Iitinois Application March 28, 1955,Serial No. 497,151 1 Claim. (Cl. 313-57) The present invention relatesto X-ray tubes and more particularly to" new and'improved cathodestructure for an X-ray tube.

Self-rectified X-ray tubes conventionally comprise a cathode and ananode mounted in facing, spaced relation within a sealed, evacuatedglass envelope. The cathode is provided with a filament for generating abeam of electrous which is focused upon a wolfram target embedded in theanode, the filament being mounted in a metal body usually designated asthe cathode head portion. The bombardment of the target by the electronsfrom the cathode lament causes X-rays to be emitted from the target,

but the continual bombardment of the target with electrons also causesit to become heated as a consequence of which it emits electrons. Theelectrons emitted from the target are usually designated as inverseelectrons and are f may melt and vaporize, the vaporized metaldepositing or condensing upon the colder parts of the tube such as theenvelope which is undesirable since such deposits impair the efficiencyof the tube and may even render a tube inoperable. Inasmuch as theintensity of the inverse elec- ,j

tron beam is directly proportional to the operating ratings of the tube,it has been necessary to operate a tube at ratings sufficiently low thatthe intensity of the inverse electron beam vlas below the point where itwould cause excessive heating and vaporizing of the material of thecathode head portion. Heretofore, cathode head portions have been formedof nickel or iron or alloy thereof and which melted and vaporized underimpingement by the inverse electron beam even though a tube was operatedat a rating considerably below that at which it otherwise could havebeen operated. Attempts have been made to protect such head portions bymounting an insert of a high melting point material such as molybdenumor woifram therein at the inverse focal spot or by providing a layer ofsuch material on the surface of the head portion. But such proceduresare expensive and even with the protective material, the possibility ofinjuring the cathode remained the limiting factor in the tube operation.It is, therefore, a principal object of the invention to provide a newand improved X-ray tube structure which reduces or negates the problemof inverse electron emission from the anode target as a limiting factorin the operable rating of the tube.

More particularly, it is an object of the invention to provide a new andimproved cathode structure for selfectied X-ray tubes which is not proneto injury upon ice bombardment by the inverse electron emission from theanode.

Still another object is to provide an inexpensive cathode structure notsubject to vaporization or melting under bombardment by the inverseelectron beam from the anode.

Other objects and advantages of the invention will become more app-arenthereinafter.

In accordance with the present invention, the tube cathode isconstructed with a head portion-of copper which it has been discoveredwill not be injured even though the tube is operated at a rating greatlyexceeding that which would be permissible with a cathode constructed inaccordance with the prior art. in fact, with a copper cathode head thelimiting factor in the tube ratingis the ability Vof the anode structureto withstand injury since the cathode head will not be injured even whenthe tube is operated at ratings which cause injury to the anode target.

For a more detailed description of the invention, reference is made tothe accompanying drawings and the detailed description thereof wherein:

Fig. l is a longitudinal, sectional view taken through an X-ray tubeconstructed in accordance with the present invention;

Fig. 2 is an enlarged, longitudinal, sectional view through the anodeand cathode of the X-ray tube;

Fig. 3 is a view looking in the direction of the arrows 3-3 of Fig. 2and showing the face of the anode; and

Fig. 4 is a View looking in the direction of the arrows r-l-d of Fig. 2and showing the face of the cathode.

The illustrated X-ray tube comprises an anode iii and a cathode l1mounted in coaxial, facing relationship within a sealed glass envelope12.

The anode 1t! comprises a copper body 14 secured in any suitable mannerto the envelope i2 and formed with a target face which is inclined withrespect to the longitudinal axis of the tube and in which face isembedded a planar faced target element l5 of Wolfram or similar materialcapable of generating X-rays upon bombardment with electrons. The faceof the anode and target element is normally inclined with respect to theaxis of the tube, as shown, so that a substantially uniform beam ofX-rays indicated at lo will be emitted laterally through the walls ofthe envelope 12.

The cathode 11 comprises a mounting sleeve 1S, one end of which isfitted telescopically over the end of a re-entrant, cylindrical glassstem portion 19 of the envelope i?. and suitably secured thereto such asby means of a plurality of turns of Wire 2Q. Mounted in the end of thesleeve 18 facing the anode lli is a massive head block or head portion22 in which is formed an electron focusing cup 23 wherein is disposed anelectron emitting filament 24. The filament 24 is adapted to beenergized through leads 2S extending outwardly through the stem 19 ofthe envelope. The head 22 may be secured to the sleeve 18 by anysuitable means such as screws 26.

As is well known, heating of the filament 24 causes the same to emitelectrons and the application of potential between the cathode 11 andanode itl causes the electrons to travel from the filament to the anode.The cathode cup 23 is shaped so as to focus the electrons upon the anodetarget in a concentrated electron beam indicated at 30, striking thetarget 15 in restricted area or focal spot 31. The impingement of theelectron beam 3) upon the target 15 will heat the target in the area ofthe focal spot 31 whereupon electrons will be emitted from the heatedportion of the target. As has been indicated hereinbefore, suchelectrons tend to leave the target in a direction at right angles to theplane of the target surface in an inverse electron beam such asindicated at 32 in Fig. 2.

lt will be understood that in the operation of a seif- X-ray Y head V22is formed as a massive body of copper.

rectified tube an alternating current potential is applied between theanode and cathode 11. Duringthe half f n the cathode, the inverseelectrons will flow from the target towards'the cathode.

The intensity of the X-rays generated atY the target of an X-ray tube isproportional to the intensity of the loading of the focal spot 31 oftarget 15, the loading ordinarily being expressed in terms ofwatts/sq.vc,m. of focal spot. The loading or tube rating may beincreased by increasing the potential lapplied between the cathode andanode, by increasing the electron generation Vat thetilament, or Vbyconcentrating the electrons ina smaller focal 'spot on the anode target.As the loading or rating on the target 15 is increased, the temperatureof the focal spot 31 will also increase, beamj32.

Ordinarily the spacing between the anode 10 and cath-Y ode 11 will besuch that the inverse electron beam 32 will strike thecathode head .22.In the tube as illustrate-the inverse electron beam' 32 impingesupon thecathode head 22 in an inverse focal spot Yadjacent they bottom ofthefocusing cup 23, the spot being indicated Vin Fig; 4 at 34. Itis'preferable, in fact, thatV the inverse beam strikethe cathode head sothat the Ybeam will not impinge upon the envelope 12 since electronsimpinging upon the envelope 12 may cause the envelope material todeteriorate or collect upon the wall,.subjecting the envelope toelectrical strain under vwhich it may fail. However', it will beapparent that similarly to the heating of the focal spot 31 onthetarget, the inverse Velectron beam 32 will 'cause the cathode head toheat at the inverse focal spot 34, vand as the intensity of the inverseelectron beam 32 increases, the heating at the inverse focal spot 34will also increase. A limiting factor in tubes constructed in accordancewith the prior art'has been't'nat the rating of the tube must be keptbelow the point where the cathode head melts and vapori/:es by reason ofthe heating at the focal spot 34. The vaporized metal condenses upon thecolder portions of the tube, and particularly upon the envelope,interfering with the operation of thetube. In many cases a tube may be.rendered completely inoperable because of the kcondensed deposits.

In accordance with the present invention, the cathode Though giving riseto a .more intense inverse Velectron cathode, to withstand injury. Thatis, in

copper has much lower melting and boiling points than 'Y any of thematerials used for cathodehead construction heretofore, boiling orVvaporization of the copper at the g Y. n

`focal point thereon of ltherinverse electronrow fromtthe anode does notoccur even when the tube is operated under ratings considerablyhigherthanpermissi'bleheretol fore since the heat Vis dispesed from-such point through the body of the head 22. As a consequence of the useof copper for constlucting Vthe increase in rating of a tube ispermissible as compared with the rating at which `a tuberas manufacturedheretofore could be operated under substantially similar conditions. lnfact, as mentioned hereinbefore, with a copper cathode head thepermissible tube loading is dependent upon the ability of the anodestructure, rather than the loading a tube the possibility of injurytothe copper cathode head portion can be disregarded and the maximumloading determined instead by the capacity of the anode torsurvive theloading since the Wolfram target of the anode ywill become overheatedbefore the cathode head is injured. inasmuch as anodes have been greatlyunderloaded heretofore, the substitution of 'a copper cathode Vheadportion makes'possible an increase in permissible loading by as much aspercent or more. While the greater heat conductivity of copper obviouslyattributes to its success, other Vfactors not at present fullyunderstood may also contribute to the outstanding improvement intherating of an X-ray tube resulting from its use. p Having illustrated anddescribed a preferred embodimentofrthe invention, it should be apparentto those skilled in the art that the invention permits of modificationsin arrangement and detail. I claim Vas my invention all suchmodifications as come within the true spirit, and scope of the appendedclaim.

I claim: Y Y V, t An X-ray tube for self-rectified operation comprisingan envelope having an anode unit and cathode. unit mounted therein vinfacing relation, .said cathode comprising a body of copper on the endthereof adjacent vsaid anode having an electron focusing cup formedtherein, 'and an electron generating lament mounted in said cup, saidbody being massive whereby heat generated at the focal point thereon ofthe inverseV electron How from said anode unit is dispersed through saidbody to prevent over heating and excessive vaporization of said bodyl atsaid focal point.

References Cited in the le of this Apatent Y.

UNITED STATES PATENTS 1,152,578 Wappler Sept. 7, 1915 1,275,542 Farrellyet al. Aug. 13, 1918 1,718,849 Bouwers June 25, 1929 2,671,867 AtleeMar. 9, 1954 Y 2,719,240 Walker Sept. 27, 1955 head 22, a verysignificant

